Problem: The scale on a map is 2cm : 9km. If the distance between two cities is 81km, how far apart in cm are the two cities on the map?
Answer: The scale means that every 2cm on the map corresponds to an actual distance of 9km. An actual distance of 81km is the same as 9 $\cdot$ 9km. The distance between the two cities on the map is 9 $\cdot$ 2cm, or 18cm.